1. Field of the Invention
The present invention is directed generally to a medical diagnostic devices that use Doppler ultrasound for obstetrical and vascular monitoring applications, and more specifically to a system for wireless transmission of signals derived from a Doppler ultrasound probe to a base unit for visual display, audible display, and recordation.
2. Description of the Prior Art
The diagnostic capabilities of the medical profession have increased significantly throughout the years. Two such advancements have been in the use of Doppler ultrasound based devices to detect and measure vascular and cardial blood flow direction and rate, to detect and measure fetal heart rate, and for numerous other diagnostic applications.
The basic Doppler effect for sound is well-known. When a source of sound and a receiver of the sound move in relation to each other, the pitch or frequency of the sound perceived or detected at the receiver is different from the pitch or frequency of the source. If they are moving toward each other, the perceived or received pitch or frequency of the sound is higher than the source sound. The classic example is standing near a railroad track as a train blowing its whistle passes. As the train approaches, the perceived whistle sound is a high pitch, which then changes abruptly to a lower pitch as the train passes and goes away from the listener.
Ultrasound is simply sound that has a higher pitch or frequency than the hearing capability of a normal human ear, which is about 20 kilohertz (KHz). The Doppler effect for ultrasound is the same as for audible sound, but, since ultrasound is at a pitch or frequency beyond the range of human ears, electronic equipment is used to detect it.
The Doppler effect is also produced in echoes, when sound or ultrasound is reflected by, or bounced off, a moving object. Thus, sound or ultrasound can be produced and projected by a speaker device or ultrasound sender, and, if it reflects or bounces off an object or target, the echo or return sound can be received and detected. If the ultrasound source, target object, and echo receiver are all stationary, the pitch or frequency of the echo ultrasound will be the same as the source ultrasound. However, if the target object is moving toward the receiver of the ultrasound echo, the ultrasound echo received and detected will have a higher pitch than if the target object was moving away from the receiver. The speed or velocity at which the target object is moving toward or away from the receiver determines the pitch or frequency of the echo received. Also, a fluid, such as blood, also reflects ultrasound waves, and the velocity or rate of blood flow determines the frequency of the echoed ultrasound waves. Thus, detecting frequencies of the echoed ultrasound waves can be used to measure direction and rate of blood flow. This Doppler effect in echoed ultrasound is the principle that is typically utilized in ultrasound medical diagnostic devices, where ultrasound signals having frequencies in the range between one (1) megahertz (MHz) and twenty (20) MHz are often used.
In medical diagnostic devices using Doppler ultrasound, the source of the ultrasound and the receiver of the ultrasound are usually transducers mounted in a hand-held probe. The probe is held relatively stationary with respect to a target object being detected or measured. Some slow movement and positioning of the probe by the physician or technician can be accommodated for detecting, if it is substantially slower than the motion of the target object. However, where accurate measurements are needed, the probe should be held quite stationary. An ultrasound wave stream is transmitted by the transducer in the probe in the direction of the target object to be detected or measured, and the return echo is received, transduced to an electric signal having both a frequency and an amplitude that corresponds to the frequency and amplitude of the echoed ultrasound waves. For example, in obstetrical applications, such as detecting or measuring fetal heart rate, the ultrasound waves from the probe are directed so as to intercept the blood flowing in a beating fetal heart. In vascular applications, the ultrasound waves from the probe are directed to intercept blood moving and circulating in a vein or artery to detect or measure blood flow and direction. In both situations, the directed signal from the probe is reflected by the flowing blood, which creates Doppler shifts from the frequency of the ultrasound by the probe to the frequencies of the echoed ultrasound reflected from the flowing blood. The reflected ultrasound echoes from the flowing blood is detected by a transducer in the probe, which converts ultrasound wave energy to electric signals. The Doppler frequency shift between the directed ultrasound and the reflected ultrasound echoes returned from the flowing blood varies proportionally with the instantaneous velocity of the flowing blood. If the blood is flowing away from the directed ultrasound from the probe, the reflected ultrasound echoes will have lower frequencies than the directed ultrasound. If the blood is flowing toward the directed ultrasound from the probe, the reflected ultrasound echoes will have higher frequencies than the directed ultrasound. Of course, if the moving target is not moving in relation to the directed ultrasound from the probe, the reflected ultrasound echo will have the same frequency as the directed ultrasound.
Doppler ultrasound techniques for medical diagnostic purposes are well known in the art. For example, see Peter Atkinson & John Woodcock, DOPPLER ULTRASOUND AND ITS USE IN CLINICAL MEASUREMENT, published by Academic Press of New York City (1982); Matthew Hussey, BASIC PHYSICS AND TECHNOLOGY OF MEDICAL DIAGNOSTIC ULTRASOUND, published by Elsevier of New York City (1985); and Peter Fish, PHYSICS AND INSTRUMENTATION OF DIAGNOSTIC MEDICAL ULTRASOUND, published by John Wiley & Sons of New York City (1990). See also, U.S. Pat. Nos. 4,276,491 issued to Daniel; 4,807,636 issued to Skidmore et al.; 4,850,364 issued to Leavitt; and 5,394,878 issued to Frazin all of which show medical devices using Doppler ultrasound techniques. Furthermore, Doppler ultrasound has become a popular method of medical diagnosis because it is non-invasive, painless, creates little or no side effects, and is relatively inexpensive. Finally, ultrasound frequencies are often used in medical diagnostic applications because they reflect well from the boundaries between different organs and blood cells without utilizing potentially harmful ionizing radiation.
In many medical diagnostic applications using Doppler ultrasound, the transmitter of the directed signal is placed directly against the human skin. For example, when measuring fetal heart rate, the transmitter is placed on the midline of the abdomen and aimed downward toward the pubic bone. When measuring vascular flow, the transmitter is placed directly over the underlying vessel. The direct contact between the transmitter and the human skin is necessary to reduce reflections of the directed ultrasound and the reflected ultrasound echo caused by the skin, and ultrasound does not propagate well in air at the frequencies used in these applications. To facilitate ease of use and manual manipulation of diagnostic devices using Doppler ultrasound, as described above, it is desirable to have a device that is small, portable, and battery operated, since the probe must often be placed directly next to the skin of the patient being tested. Current Doppler ultrasound probes are connected by a cord containing electric wires to a base unit, where the electric signals from the receiving transducer are processed for display in useful information format. While such current devices are very useful and effective, there has been a need for even further improvements-one of which is to eliminate the cord between the probe and the base unit. A cordless probe would make the probe easier to handle and use, and it would reduce the amount of equipment placed in the sterile field around the patient. A cordless probe would also enable the person using the diagnostic device to place the probe in any desired position, unencumbered by a cord and the positioning restrictions that a cord might impose. There are some constraints, however, in replacing the cord with some kind of wireless signal transmission system between the probe and the base unit. For example, the signal transmitted from the probe to the base unit cannot interfere with other medical equipment in the room, hospital, or ambulance. The signal transmitted from the probe to the base unit may also interfere with the operation of the Doppler transceiver, itself, if the harmonics coincide with the Doppler signal. Furthermore, since Doppler ultrasound in an obstetrical application is often used to reassure the mother of the presence of fetal life, it is crucial that the signal from the probe be received by the base unit in a very reliable manner, regardless of its position in relation to the base receiver and to other objects and persons in the room, to avoid alarming the mother. Finally, it is desirable to have the batteries used in the probe and the base unit be rechargeable during storage of the base unit and the probe so that the batteries will be fully charged when the probe and the base unit are used.